Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia. It affects 1% to 2% of the general population with an important increase in incidence with age. In the United States it is estimated that over 5 million people have atrial fibrillation, and because of our aging population the prevalence of this arrhythmia will increase significantly over the next decade.
Atrial fibrillation is associated with increased morbidity and mortality, and in particular, a general decrease in quality of life for those afflicted with atrial fibrillation. AF can also cause tachycardia mediated cardiomyopathy or worsening of pre-existing heart failure. Moreover, AF is known to increase the mortality risk 1.5-2 fold with the risk for stroke five-fold. Patients are at an increased risk of stroke unless they are treated adequately with anticoagulants. Anticoagulant treatment however, increases the patient's risk of bleeding, which carries with it is own set of dangers. Medications currently available for treating atrial fibrillation have proven to be only moderately effective in decreasing the incidence of recurrent atrial fibrillation, and these medications do not decrease the patient's risk of having a stroke.
One method of treating atrial fibrillation has been to perform ablation of selected areas of the left atrium. There is strong evidence to suggest that ablating these areas of the left atrium serves to cure or prevent further incidences of atrial fibrillation, which thereby has shown to reduce the risk of stroke and reduce the necessity of anticoagulant therapy. Typically, ablation of this type is carried out via an intravascular catheter using radiofrequency or microwave energy to cause thermal changes to the selected parts of the left atrial tissue.
Besides having a good safety profile, catheter ablation therapy for AF has proved effective in establishing and maintaining sinus rhythm. Ablation for atrial fibrillation is now the most commonly performed procedure in most laboratories.
The posterior wall of the left atrium is particularly targeted for ablation because the pulmonary veins enter the atrium at this area of the left atrium, encircling the pulmonary veins with continuous rings of lesions in this procedure. The esophagus may however be, in a position so as to overlie one or more of these circles, thereby making the desired encirclement difficult or impossible.
A significant and lethal complication of atrial fibrillation ablation is the accidental creation of an atrial esophageal fistula following the development of lesions on the posterior wall of the left atrium. Because the esophagus is generally in close position to the posterior wall of the left atrial, thermal injury may be communicated to the esophageal wall resulting in disruption of the wall and formation of the atrial esophageal fistula. Thermal esophageal lesions are believed to be precursors of fistula formation. Post ablation esophageal wall changes (erosion or ulceration) are reported to occur in up to 47% of patients. Real time temperature monitoring can detect rapid esophageal heating during radiofrequency ablation.
Although the pathophysiology of left atrial-esophageal (LA-Eso) fistula formation is not fully understood, it is clear that thermal injury to the esophagus during ablation of the LA posterior wall plays a crucial role in triggering the cascade of events that eventually result in the development of LA-Eso fistula.
Currently, the most commonly used clinical strategy to minimize esophageal thermal injury during AF ablation involves limiting the magnitude of power 25 to 35 W, as well as the duration (<30 s), of RF applications placed along the posterior wall of the LA. A major limitation of this approach is that it fails to account for the variability in the thickness of the posterior LA wall and the presence of peri-esophageal connective tissue—important determinants of esophageal heating. Thus, empirically limiting the power and duration of RF applications may be insufficient to prevent esophageal thermal injury in all patients. RF power delivery during AF ablation, guided by luminal esophageal temperature (LET) monitoring is associated with less frequent esophageal injury compared with a strategy of power limitation alone.
Also, it is known that successful atrial fibrillation ablation may require the introduction of lesions near the location of the inferior right pulmonary vein, which is located in close proximity to the phrenic nerve. Thus, it has become more common for accidental injury to the phrenic nerve to occur. The phrenic nerve is responsible for operation of the diaphragm, and thus, injury to the phrenic nerve can be quite catastrophic.
Luminal esophageal temperature (LET) monitoring is the most common strategy to minimize esophageal injury during atrial fibrillation (AF) ablation procedures. The esophageal probe may have one thermistor, or the esophageal probe may have multiple sensors on the body of the probe for measuring temperature from a length of the esophagus.
In addition to the foregoing, fractionated electrograms and vagal plexi are also frequently present on the posterior wall of the left atrium. These are also common targets of atrial fibrillation ablation. Again, the location of the esophagus may hinder application of this sufficient energy to successfully ablate enough energy of the left atrium to prevent recurrence of atrial fibrillation.
Since esophageal injury during RF ablation in the left atrium is thermal injury, and because of the need for preventing injury to the esophagus, there is a real need for a method and system for,                a) activating various levels of alarms based on esophageal temperature monitoring,        b) cooling the esophagus, and/or        c) automatically interrupting the energy delivery of the ablation circuit,whenever the esophageal temperature reaches a predetermined critical level.        